When we think about how things move in nature, we usually think of the path of least resistance. Water flows downhill. Smoke rises into the air. A dropped pen falls to the floor. In biology, this passive movement is called diffusion —particles naturally shifting from an area of high concentration to an area of low concentration until they reach a balanced state.
If passive transport is like rolling a ball downhill, active transport is like throwing that ball back up the hill—every single time. It requires energy, specialized machinery, and constant effort. define active transport
Because this movement is "uphill" (moving from low to high), it cannot happen spontaneously. The cell must expend energy, usually in the form of , to power the process. When we think about how things move in